Modification of the Gelfand-Levitan Method for 1-D Multylaered Structure Inverse Problem

For dielectric slab with step profile of dielectric constant the Gelfand-Levitan method is correct if peaks of time-domain reflected signal are close to δ-pulses. Combination of parametric spectral methods for obtaining time-domain signal from frequency domain data and Gelfand-Levitan method for time-domain signal processing can help to improve the solution of the problem. Results of numerical simulation are presented.Для диэлектрической плиты со ступенчатым профилем диэлектрической постоянной применим метод Гельфанда-Левитана, если пики отраженного сигнала близки к δ -импульсам. Комбинация параметрических спектральных методов для получения сигнала во временной области по данным из частотной области и метод Гельфанда-Левитана для обработки сигнала во временной области позволяют получить усовершенствованный алгоритм решения задачи. Приведены результаты численного моделирования.Для діелектричної плити зі східчастим профілем діелектричної сталої метод Гельфанда-Левітана застосовний, якщо піки відбитого сигналу близькі до δ - імпульсів. Комбінація параметричних спектральних методів для отримання сигналу в часовій області та метод Гельфанда-Левітана для обробки сигналу в часовій області дозволяють отримати удосконалений алгоритм розв’язання задачі. Наведено результати чисельного моделювання

Bosonic Excitations in Random Media

We consider classical normal modes and non-interacting bosonic excitations in disordered systems. We emphasise generic aspects of such problems and parallels with disordered, non-interacting systems of fermions, and discuss in particular the relevance for bosonic excitations of symmetry classes known in the fermionic context. We also stress important differences between bosonic and fermionic problems. One of these follows from the fact that ground state stability of a system requires all bosonic excitation energy levels to be positive, while stability in systems of non-interacting fermions is ensured by the exclusion principle, whatever the single-particle energies. As a consequence, simple models of uncorrelated disorder are less useful for bosonic systems than for fermionic ones, and it is generally important to study the excitation spectrum in conjunction with the problem of constructing a disorder-dependent ground state: we show how a mapping to an operator with chiral symmetry provides a useful tool for doing this. A second difference involves the distinction for bosonic systems between excitations which are Goldstone modes and those which are not. In the case of Goldstone modes we review established results illustrating the fact that disorder decouples from excitations in the low frequency limit, above a critical dimension $d_c$, which in different circumstances takes the values $d_c=2$ and $d_c=0$. For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as $\rho(\omega) \propto \omega^4$ is a universal feature in systems with ground states that depend on the disorder realisation. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension

Measurement of the neutron lifetime with ultra-cold neutrons stored in a magneto-gravitational trap

International audienceWe report a measurement of the neutron lifetime using ultracold neutrons stored in a magneto-gravitational trap made of permanent magnets. Neutrons surviving in the trap after fixed storage times have been counted and the trap losses have continuously been monitored during storage by detecting neutrons leaking from the trap. The value of the neutron lifetime resulting from this measurement is τ$_{n}$ = (878.3 ± 1.6$_{stat}$ ± 1.0$_{syst}$) A unique feature of this experiment is the monitoring of leaking neutrons providing a robust control of the main systematic loss

Magnetic storage of UCN for a measurement of the neutron lifetime

International audienceWe present the status of an experimental setup designed and built for the measurement of the neutron lifetime and we describe also details about the measuring sequence. The central element of the setup is a magnetic trap made of permanent magnets. Neutrons surviving in the trap after fixed storage times are counted after their extraction and the trap losses are continuously monitored during the storage periods. The technique has achieved a statistical sensitivity of about 2 s on the storage time what constitutes the most sensitive magnetic trapping technique for ultra-cold neutrons developed so far